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During the process of selection, individuals having disadvantageous traits are weeded out. If the selection pressure isn't strong enough then mildly disadvantageous traits will continue to persist in the population. So the reasons for why a trait is not evolved even though it may be advantageous to the organism, are: There is no strong pressure against ...


38

Note: This is an answer to the last line of your question. A classical example of animals adapting to the influence of humans on their environment is the adaption of the Peppered Moth. Here is a brief summary: The peppered moth was originally a mostly unpigmented animal (<1800). During the industrial revolution in the southern parts of the UK a lot of ...


34

There's always the most obvious: Evolution is chance. Some traits allow an individual to have a higher chance to produce offspring. That doesn't mean individuals with that trait have more offspring, not even on average unless the law of large numbers applies. A randomly mutated perfect squirrel could appear, and since it's only one, it gets run over by a ...


34

Short answer The phenomenon you describe can be explained by the negative afterimage effect, which indeed is elicited by adaptive processes in the retinae. Background In the retina there are three types of color photoreceptors, called cones. They are sensitive to red, green and blue, respectively. Colors are processed in the retina through a process ...


24

Many insects (as well as some other animals) have documented resistance to pesticides. For example, the German cockroach (Blattella germanica) can be resistant to multiple insecticides1. In addition, some populations of this cockroach are now repelled by glucose, which leads to them avoiding traps2. References: 1: Fardisi, M., Gondhalekar, A. D., Ashbrook,...


19

Because evolution is an effect, not a cause. That is, there's no "God of Evolution" out there deciding that this or that trait would be beneficial to a species, and deciding to add it. Evolution just works* on whatever random variations happen to come along. *And as others have pointed out, it works statistically, not deterministically.


16

The technical answer is: Because the coloration of skin and hair is done by the two forms of melanin: Eumelanin, which is dark brown to black and Pheomelanin which is yellow to red. This enables colors from white (not pigmentation) to black (dense eumelanin pigmentation) and also colors in between by different ratios of the two pigments. The evolutionary ...


16

Bighorn sheep are developing smaller horns and elephants are becoming tuskless in Africa: The horns of some bighorn sheep are getting smaller, because hunters are picking off the most impressive rams before they reach their breeding peak Elephant poaching, for example, is thought to have led to an increase in the number of tuskless animals in Africa....


15

Below are the reasons I can think of. The list is not exhaustive and there are some conceptual overlaps. The trait seems advantageous but it is not, maybe due to its effect on another component of fitness (trade-off). It sounds to me to be the most likely explanation whenever you are wondering why a given species does not carry a given trait. In other words,...


14

A likely misunderstanding of yours Now we almost don't fight with other species Misunderstanding about selection As you will go through this course, you will understand why this sentence makes little sense. A change in allele frequency via natural selection is caused by a fitness differential among genotypes within a population. The existence of a ...


11

Evolution occurs by a change in gene frequencies, with gene frequencies potentially affected by four mechanisms (mutation, migration, drift, and selection). The answer to the question Why does the seemingly advantageous trait X not evolve? could be: The mutations for a trait have never occurred within a population, or such genes have never migrated into ...


11

Every species on the planet is "transitional" - this is because there is no ultimate or final species. Species branch out from one another, sometimes species go extinct, leaving gaps between the extant branches. But it also comes down to how you look at it; if you were comparing blue whales and humans, then chimpanzees (and many other species) would be "...


9

Apparently the most noticeable research on this subject of a rabbit's white behind has been done by Dr Dirk Semmann of the University of Goettingen. He proposes that these spots actually confuse predators because of their very noticeable nature. By focusing on the bright spot, the would-be predator ignores the larger body of the animal. Then, when ...


9

The question is probably more complicated than it seems because, if I am not wrong, the word adaptation here is understood at the group level. Definitions of adaptation Unfortunately, there is not such thing as a single, standard definition of adaptation. But for most cases the accurate definition the author is using is not of much importance as all of the ...


9

Just to add a different dimension to the answer from @Chris. Not all animal colouration is produced by melanin. A whole range of bright colours in insects, birds and reptiles comes under the heading of structural colouration, which basically involves having a repeating structure at the microscopic level to interact with light. This is the basis for macaw ...


9

Good question. And good analysis. I have little to add! I'll simply provide my own list of thoughts to complement your ideas, which are not mutually exclusive. The fact that it wasn't discarded during the course of the species' evolution suggests it must have offered some benefit. This statement is speculative. The key word here is suggests. i can ...


8

Let's start with your definition. "Selection for traits that would be beneficial to a population of units at the expense of an individual unit possessing the trait" This is not a good definition of group selection. In reality, selection can act on groups regardless of the direction of selection at the individual level. This definition sounds to me (your ...


7

Has anyone calculated or simulated how the adaptation for many different traits can happen simultaneously? There are a lots of studies on the subject but I don't fully understand what is your issue. So I'll try to give some words hoping that helps a bit but it is possible that I'll totally miss the point you want to make. The mutation rate in humans is ...


7

Standing genetic variation is when there is more than one allele at locus in the population at the time-point in question. When an allele goes to fixation there is no standing genetic variation at the locus until new mutations occur. Loci where alleles are not fixed are described as having standing genetic variation. "Standing genetic variation: the ...


7

A trait is said to be adaptive when it causes fitness to increase. Fitness is generally understood as the (relative) contribution to future generations in terms of offspring or genes. The trait is selected for by the environment and hence increases fitness. In the paper of Dey et al. this is the fitness of the parent birds. Hatching asynchrony causes size ...


7

A dimension not explored by the other (excellent) answers has to do with color perception under trees. Leaves are green while on the tree, which tends to make mostly green light available to the understory. Viewed under green light, a green-furred animal would appear bright green, roughly the same as a white creature viewed under green light. A red or ...


7

Albeit used in the keywords of the paper, feather pecking is not an addiction, but normal behavior turned pathological in artificial (overcrowded) environments. Addiction typically arises through overstimulation of reward centers in the brain, most notably the dopaminergic pathways involving the nucleus accumbens. Massive dopamine release, e.g. through ...


7

First of all what a nice foto! I think this is Antheraea polyphemus. According to Wikipedia, the moth has an average of 15 cm (6 in). The purplish eyespots on hind wings give its name - from the Greek myth of the Cyclops Polyphemus. And about the defence strategy it's quite interesting. As a green caterpillar, it is camouflaged by its color. However, if ...


6

For example sundews are plants (so autotrophic), but they "hunt" for insects to get additional nutrients, e.g. nitrogen. As far as I remember the nitrogen is the main reason for eating other organisms but they also use other substances, including carbohydrates, from their prey.


6

The study you are referring to is probably a study by Culver et al. (2000). They performed a genetic analysis of 315 individuals of the American Puma (Puma concolor), also known as the cougar. Their thorough analysis (especially for the late 1990s) showed that the Florida panther and the western North American pumas were the same subspecies, Puma concolor ...


6

From your own PlosOne link comes proof that the selection pressure was very weak: "Most locations (59%, n = 37) were sampled in only one year, 20 locations in two years, five locations in three years, and one in four years, yielding in total 96 unique location-year combinations of measurements of seasonal total flying insect biomass." "We collected in total ...


5

Your questions mean basically the same. Birdcare.com says: The situation in which all the eggs in a clutch do not hatch at (more or less) the same time, as is more usual among birds, but have their hatching spread over several days. It is well seen in the various types of raptor, and is an adaptation to a type of food supply which may fluctuate. During ...


5

Just look up kleptoplasty http://en.wikipedia.org/wiki/Kleptoplasty The slugs who most frequently employ it would fit your more restrictive definition. Now, if we're getting technical, they aren't born that way. They steal the chloroplasts, but you didn't say, so I guess you need to revise your expectations again. ;)


5

The closest reason that I know of for such behaviour is simple conditioning! B. F. Skinner conducted some fairly famous experiments with pigeons, where he put hungry pigeons in cages, and randomly administered the food. The pigeons associated the foods arrival with whatever they happened to be doing at the time (be it looking to the right, or bobbing their ...


5

In sexually dimorphic ant-mimicking spiders, it depends on the specific species which sex resembles the ant most (Cushing, 2012). In many cases of sexually dimorphic spider myrmecomorphs, the male is more mimetic than the female, such as in Corinnidae species and the genus Castianeira, Oonopidae and Antoonops. Such sexual dimorphism may be adaptive if the ...


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